1. Field of the Invention
The present invention relates generally to a method and apparatus for ammonia recovery, and more specifically to such a method and apparatus for recovering pure ammonia from a gaseous solution.
2. Discussion of the Related Art
Ammonia is a major component of modern industrial fertilizer production. As disclosed in "Ammonia Catalysis and Manufacture," A. Nielson ed., Springer-Verlag Berlin Heidelberg, NY, N.Y. (1995), the most common industrial process for ammonia synthesis involves reforming a hydrocarbon fuel with air and steam to form a mixture of gaseous nitrogen and hydrogen in a 1 to 3 molar ratio. This gas mixture is then compressed to high pressure (e.g., about 80 bar to about 220 bar), heated (e.g., about 450.degree. C. to about 550.degree. C.) and passed over a catalyst where ammonia formation occurs. During ammonia synthesis, the reactants (i.e., nitrogen and hydrogen) and the product (i.e., ammonia) are in equilibrium, so, to increase the total amount of ammonia formed, the equilibrium should be shifted to product formation by removing ammonia from the reaction mixture as it is produced.
Removal of the ammonia is usually accomplished by cooling the gas mixture to a relatively low temperature (e.g., about -5.degree. C. to about 25.degree. C.). In this temperature range, a two-phase mixture is formed with ammonia being a liquid and nitrogen and hydrogen remaining as gases. The liquified ammonia is separated from the other components of the mixture, and the remaining nitrogen and hydrogen are subsequently re-heated to the operating temperature for ammonia conversion and passed through the reactor again.
There are two major disadvantages to this technique of ammonia isolation. First, the ammonia separation is often incomplete, so the gas that is recycled to the reactor can include a substantial amount of ammonia. This decreases the efficiency of ammonia production. Second, cost reduction for ammonia production by this method is limited by the fact that a substantial amount of the energy used in this process is related to cooling the gas mixture to separate ammonia.
A further complication with this method involves the presence of oxygen-containing species in the synthesis gas mixture during ammonia production. These oxygen-containing species, especially carbon monoxide, can poison the catalyst and reduce the efficiency of ammonia production. To remove the oxygen-containing species, they can be passed through a methanation catalyst prior to entering the ammonia synthesis loop. This converts the oxygen-containing species into inert species, such as methane, that are less likely to poison the catalyst. However, the resulting species dilute the reaction mixture which can decrease ammonia production capacity. To overcome this problem, the oxygen-containing species can be removed from the reaction mixture, but this increases the cost associated with ammonia production.
Distillation and single-stage flash are other methods that have been used to separate ammonia from a synthesis gas. However, similar to the above-noted technique, distillation and single-stage flash also involve the complexities associated with the separation of certain inert species from the reaction mixture.
U.S. Pat. No. 4,537,760 discloses a process for isolating ammonia from synthesis gas in which beds of sorbent are used to remove ammonia from a recycled synthesis gas subsequent to ammonia condensation. The ammonia is desorbed from the sorbent beds by the hot gases from the reactor. The desorbed ammonia is condensed at reduced temperature and phase separated as liquid. The sorbent beds are then regenerated by a counter current flow of a hot synthesis gas from the converter before a refrigeration and condensation step. While this process can improve the efficiency of ammonia production, this improvement is comparatively modest because the gases still must be cooled to a low temperature.
"Separation of Gases by Pressure Swing Adsorption," R. V. Jasra et al., Separation Science and Technology, volume 26(7), pages 885-930, (1991) and U.S. Pat. Nos. 2,944,627, 3,069,830, 3,237,377 and 3,237,379 disclose a method for gas purification known as pressure swing absorption. In this process, water vapor in high pressure air (e.g., 150 p.s.i.a.) is absorbed onto a sorbent in a bed, and then the bed is depressurized. A volume of dry air at low pressure is passed through the sorbent bed in the opposite direction to remove the adsorbed water from the sorbent bed. This process can be effective at drying air because the vapor pressure of the water sorbed onto the sorbent is nearly independent of the pressure of the air contacting the sorbent. While pressure swing absorption may be advantageously used in some systems, this process would be uneconomical in ammonia production due to the large volumes of synthesis gas recycled in ammonia production.
As seen from the foregoing discussion, it remains a challenge in the art to provide a method and apparatus for ammonia production and isolation that provides improved efficiency and reduced cost. In particular, it would be desirable to provide such a method and apparatus that could cut the cost and avoid the complexity of ammonia condensation. Furthermore, it would be advantageous if the method and apparatus could be performed without substantial temperature and/or pressure reductions.